Methods to understand and map underground deep fluid (water or oil) flows employ a wide range of technologies; however, the most successful approaches utilize a tracer (or taggant) that is selectively placed deep within the well and detected at the well-head upon elution. See U.S. Pat. No. 3,002,091 to F. E. Armstrong; U.S. Pat. No. 4,264,329 to J. R. Beckett; U.S. Pat. No. 4,742,873 to F. F. Craig; U.S. Pat. No. 3,993,131 to E. F. Reiedel; and S. K. Ritter, C&E News 92, 31 (2014). While taggant methods are useful, they suffer from a variety of limitations, with the biggest concerns being the relatively short (2-3 mo.) time of use and the limited number of unique taggants available.
Hydrocarbon-soluble compounds are being developed that can be intercalated into porous proppants and then sealed in with a polymeric coating. After placing the proppants in the appropriate underground locations during the drilling process, these soluble taggants, over time, gradually diffuse through the polymeric coating and are released into the bulk underground reservoir. Ideally, the soluble molecules are transported to the well-head, where routine sampling using simple analytical tools would be used to identify them. While the proppant coating will be used to control the time of release, the tracer molecules themselves must fulfill a variety of criteria, including being soluble in the various underground fluids of interest, surviving high temperature and pressures, be uniquely identifiable in trace quantities, and number in the 50-100+ range. Therefore, a series of salen-metal complexes have been explored due to their reported stability to high temperatures and pressures. (salen is a contraction for salicylaldehyde and ethylenediamine. H2-salen is made by the condensation of salicylaldehyde and ethylenediamine). See T. J. Boyle et al., Inorg. Chem. 57(5), 2402 (2018); Z. F. Dai et al., J. Cat. 338, 202 (2016); Q. Y. Meng et al., J. Poly. Sci. A—Poly Chem. 54, 2785 (2016); J. Rakhtshah et al., J. Coord. Chem. 70, 340 (2017); A. Rezaeifard et al., Rsc Advances 6, 64640 (2016); and C. Sohn et al., Dalton Trans. 45, 5825 (2016). Further, modification to the electron rich rings of the salen ligands allows for tuning of the salen-metal complexes' solubility and identifying vibrational spectroscopic signature. When these modified ligands are combined with different metals, a nearly unlimited number of easily distinguishable taggants can be produced that should survive the underground environment of interest.
Numerous group 13 salen compounds have been structurally identified (>110 structures) and used for a variety of applications, including ceramic materials, light emitting diodes, antimicrobial agents, and polymerization catalyst; however, none have been applied to down-hole fluid flow tracking. See R. M. Clarke and T. Storr, Dalton Trans., 9365 (July, 2014); F. S. Nworie et al., J. Bas. Appl. Res. 2, 295 (2016); M. A. Musa et al., Lett. Drug Desg. Disc. 7, 165 (2010); D. A. Atwood and M. J. Harvey, Chem. Rev. 101, 37 (2001); P. G. Cozzi, Chem. Soc. Rev. 33, 410 (2004); S. Dagorne et al., Coord. Chem. Rev. 257, 1869 (2013); and D. Specklin et al., Dalton Trans. 46, 12824 (2017). However, no structure reports concerning Tl-salen derivatives have been published. While there are a number of M(I)-salen structures available, most of these are heterometallic derivatives forming alkali metal, silver, chromium or copper salts, or unusual oxo derivatives employing the alkali metals (A=Li, Na, K). See S. C. Ball et al., J. Chem. Soc.—Chem. Commun., 2147 (1995); I. Correia et al., Eur. J. Inorg. Chem., 732 (2005); G. B. Deacon et al., Inorg. Chim. Acta 360, 1364 (2007); R. Jia et al., Aust. J. Chem. 69, 20 (2016); and E. Solari et al., J. Chem. Soc., Dalton Trans., 2471 (1991). Cyclooctadiene rhodium(I) salen is the only homometallic M(I)-salen complex reported, where two Rh cations bind to different O and N atoms of the salen backbone. See C. Janiak et al., Dalton Trans., 3698 (2009). Note: a Rh(I)-salen derivative is also reported but the metal does not interact with the O or N atom of the salen but rather with a phosphino/thio moiety located off of the cyclohexyl salen backbone. See M. S. I. Masar et al., J. Am. Chem. Soc. 129, 10149 (2007).
Therefore, a need remains for structurally characterized salen derivatives for the heaviest congener, thallium.